It depends on what you're trying to do, i.e. what is your definition of fast? I and some other geezer folks I ride with are into the Death Ride and similar pursuits. Volume is key, right along with intensity, maybe twice a week. Which basically means conditioning, because it keeps getting harder to keep the volume up and still be able to do the intensity and vice versa. I've been doing 11 hours/week.
73 y.o. I've been riding tandem with my wife, but she was on the mildly injured list this weekend, so I rode my single with the faster boys, 10-20 years younger. Dropped me on every hill, but I kept getting them back. 1:06 of Z4 in 66 miles. We didn't have hills bigger than about 700', so my program was 15' of Z4, recover a little, hit it again. I don't do HIIT. Snow's melting. First pass ride of the season next Sunday.
I don't thing anyone ever advocated long slow rides. They just make you slow. Long steady rides OTOH are exhausting. If they aren't, you're going it wrong. 3-4 hours should pretty much have you wishing it was over. I see some riders coasting or taking it easy on the descents or downwind flats. Don't do that. Ride like you're on your rollers, holding steady zone 2. Never let off the pedal pressure.
Tandem makes me strong. It's not true that you can get just as strong on a light bike. You want to get strong, ride tandem for 60 miles of hills with your wife every weekend, whether she's a cyclist or not. Fix you right up, oh yeah..
I've been lifting weights for about 20 years. My periodization has gradually evolved. During the season, which is already here, I'm trying to do one session of 50 minutes per week, like 2 sets of 4-5 reps, max on the last set. OTOH, if it has to be between 2 X 20 or weights, I do the 2 X 20. I do the weights after an hour of so of Z2 or pedaling drills on the rollers. Of course I do more weight work in the winter, but never more than 2 hours/week. More than that, you're doing it wrong.